Bumble bees and other pollinators feed on nectar and pollen, with pollen considered the primary source of protein critical for rearing young. Recent studies reveal that both nectar and pollen are replete with microbes and that bumble bees prefer yeasty nectar over yeast-free nectar. However, why bumble bees seek out yeasts is not clear. Other bees feed larvae fermented pollen, suggesting that yeast might provide additional protein to developing young. We hypothesized that bumble bees cultivate yeasts on pollen to enrich protein provisions for developing young. To test this hypothesis, we first verified active yeast growth (via a plating assay) from comb and pollen pot swabs of commercial bumble bee colonies (B. impatiens). We then determined how yeast growth on pollen affected total protein content of what would be the larval diet by inoculating sterile artificial pollen with a yeast strain isolated from comb. Yeast-inoculated pollen had a ten percent higher protein concentration than sterile pollen immediately after inoculation and two-fold higher protein after incubation in colony conditions for 24 hours, revealing dramatic effects of yeast on quality of larval food. Ongoing work will aim to determine whether this is simply a passive outcome of a ubiquitous microbe being unwittingly transported into bumble bee colonies, or whether bumble bees actively cultivate yeast as a high quality protein source for developing larvae.